1. Field of the Invention
The present invention pertains to an apparatus employed in the delivery of a flow of breathing gas to the airway of a patient, and, more particularly, to an exhaust assembly adapted to provide a controlled flow of exhaust gas over a range of supply pressures.
2. Description of the Related Art
A ventilation circuit, also commonly referred as a breathing circuit or a patient circuit, may be employed to supply a continuous flow of breathing gas to the airway of a spontaneously breathing patient. The ventilation circuit may include an exhaust assembly which provides an exhaust flow path for air exhaled by the patient. During the exhalation phase of the patient's breathing, exhalation gas backflows into the ventilation circuit and, unless exhausted from the circuit, is re-breathed by the patient during the next inhalation phase. Re-breathing of exhaled gas is undesirable because the exhalation flow contains carbon dioxide (CO2). Reliable and thorough exhaustion or purging of exhalation gas from the ventilation circuit is, thus, an important feature of such ventilation circuits. This purging becomes a more difficult problem the further the exhalation valve is located from the patient interface due to the amount of residual exhaled gas in the intervening ventilation circuit dead space.
Conventional exhalation assemblies provide varying fluid flow at varying fluid pressures. However, the naturally occurring relationship of fluid flow rate to pressure (flow is proportional to the square root of pressure) may be unacceptable in a ventilation circuit for a number of reasons. In medical ventilators used to assist patient breathing and in similar devices used to supply breathing gas to a spontaneously breathing patient, for example, it has been common practice to include a fixed size leak, especially in single limb patient circuits. The size of the leak is selected such that the exhaust gas is flushed out of the patient circuit before the exhaled gas can be re-breathed by the patient with the next inhalation. The fixed sized, i.e., fixed geometry or fixed orifice, leak may be provided by an exhalation valve. However, an exhalation valve with a flow characteristic that varies naturally with pressure variations in the patient circuit may not be suitable as a patient circuit exhaust valve.
Additionally, some pressure support systems are adapted to supply bi-level pressure support in which the pressure of the air supplied to the patient during exhalation is lower than the pressure of the air supplied to the patient during inhalation. With many conventional exhalation valves, the size of the fixed leak needed to flush exhaled gas from the circuit under the relatively lower pressure of exhalation is very large. However, the flow rate through such a fixed leak at the higher inhalation pressure results in enormous waste of supply gas during inhalation. Such conditions may require a larger capacity pressure generator, commonly a blower, for example, than would otherwise be required. This too is wasteful in that the result is unnecessary added cost for the apparatus and increased energy consumption to operate it.
It is, therefore, preferable that the flow rate of the leak provided to flush exhaled gas from such a ventilation circuit does not change appreciably within the range of pressures applied in the circuit. In this sense, a fixed leak is more desirably one that provides a fixed flow rate, than one characterized by a flow path cross-sectional area of fixed size.
Among the prior art of valves purporting to regulate flow by means of pressure actuated regulators are those disclosed in U.S. Pat. Nos. 3,467,136, 3,474,831, 3,592,237, 3,948,289 and 3,951,379. Other flow regulating valves are disclosed in U.S. Pat. Nos. 3,429,342, 3,473,571, 3,770,104, 4,182,371, 4,234,013, 4,280,527, 4,351,510 and 4,354,516. U.S. Pat. No. 4,428,397, apparently related German Patent no. DE 27 48 055 A1, and Russian Patent abstract SU 1015344A disclose a valve for controlling the rate of flow of fluid therethrough. In addition, U.S. Pat. No. 5,002,050 discloses a medical gas flow control valve and U.S. Pat. No. 5,438,981 discloses an automatic safety valve and diffuser for a nasal and/or oral gas delivery mask.
U.S. Pat. Nos. 5,685,296 and 5,937,855 disclose a flow regulating valve that exhausts gas from a ventilation circuit at a constant flow rate despite varying pressure in the ventilation circuit. However, as noted above, it may be further desirable to locate the exhaust valve at the mask, for example, so that the amount of ventilation circuit dead space is minimized and so that the need to provide a relatively bulky exhaust structure on the ventilation circuit is eliminated. In treating obstructive sleep apnea OSA, for example, a positive pressure therapy is provided to the patient while he or she sleeps. Therefore, it is preferable for the patient circuit to be flexible and readily movable during the pressure support treatment. This goal may be frustrated by providing the exhaust valve on the patient circuit. In the hospital setting, it is also preferable to minimize the entanglements on the patient circuit to keep as clear a working area as possible for the doctors and nurses.
Accordingly, a need exists for an apparatus and method for providing improved comfort for a patient receiving a flow of breathing gas which overcomes these and other problems associated with known systems.